Regardless of how old or new your BMW is, at some point, you will experience a glow plug failure. Like any diesel car, your BMW has four glow plugs (or six on six-cylinder models), which preheat the combustion chamber to aid in starting on cold mornings. BMW also employs an afterglow feature, allowing the glow plugs to run for a few minutes after starting to help reduce emissions.
Many owners don’t realize that if their vehicle has a DPF (Diesel Particulate Filter), a failed glow plug will prevent the DPF system from regenerating. This can eventually lead to expensive issues with the particulate filter. Therefore, it is wise to replace faulty glow plugs as soon as the failure is diagnosed. If you're unsure whether you have a glow plug failure, you can read and reset error codes using our diagnostic software package.
It is also a false economy to replace only the failed glow plug(s). Instead, it is better to replace the entire set. The reasoning behind this is that accessing the glow plugs requires removing several plastic covers and parts, a process that takes considerable time. Since all the glow plugs have been in use for the same duration, it is likely that the remaining ones will fail soon, requiring another round of labour-intensive work.
When replacing glow plugs, it is crucial to use high-quality, long-life glow plugs. I highly recommend using the BERU brand.
Removing Seized Glow Plugs
One of the biggest challenges when replacing glow plugs in any diesel engine is the risk of them seizing in the cylinder head. If they snap, it may require either cylinder head removal or a specialist to extract the broken plug in situ. While such services are now more affordable than before, it is best to avoid this situation altogether.
Glow plugs often seize due to carbon buildup or corrosion between the glow plug body and the cylinder head. To minimize this risk, exercise great care when removing them. Soaking the plugs overnight in a penetrating fluid can make them easier to extract the following day.
When removing glow plugs, I always assume they will be stuck. One of the best ways to loosen corrosion is by soaking the glow plugs overnight. Kano Kroil is an excellent penetrating oil that seeps into the glow plug threads and breaks down carbon and corrosion buildup, making removal much easier. If Kano Kroil is unavailable, I recommend a 50/50 mix of ATF fluid and acetone or a 50/50 mix of Coke and vinegar as alternatives.
Even after soaking, patience is key. Work slowly and steadily on each glow plug. Applying excessive torque too quickly can cause breakage. A good method is to slightly tighten the plug first to help break any corrosion seal on the threads, then gently apply steady pressure to loosen it. Even if the plug starts to come out, take your time—work it back and forth carefully. If resistance is felt, stop and soak it again before continuing.
As the glow plug loosens, I apply Brunox Turbo Spray into the threads, giving a good spray every turn. If any glow plugs still feel tight while unscrewing, I soak them again for an hour in Kano Kroil and take a break. Impatience can result in snapped plugs, leading to delays and costly repairs. However, I have yet to encounter a BMW glow plug that could not be removed with Kano Kroil, Brunox Turbo, and a bit of patience.
Recommended Penetrating Fluids
Avoid using WD-40, as it is not the best penetrating fluid for stuck glow plugs. If the recommended fluids are unavailable, the following products can also be used for pre-soaking and removal:
50/50 mix of Coke and vinegar (can also be used separately)
50/50 mix of mineral ATF fluid and acetone
PlusGas Spray
Brunox Turbo
Liquid Wrench Penetrating Spray
BG In-Force Penetrating Spray
Preventing Future Seizure
When installing new glow plugs, apply high-temperature ceramic grease to the threads. This prevents them from seizing in the head if they need to be removed in the future. Do not use conventional copper grease, as it is unsuitable for this application.
After replacing the glow plugs, you must clear the fault codes using diagnostic software to ensure proper operation.
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